Amplifiers traditionally have been linear, including various variations of class A, class B and class AB, with power output transistors acting as linear regulators to modulate the output voltage. A recent development in high efficiency amplifier design is the class D amplifier, which is basically a switching amplifier with the switches either fully on or off, thus significantly reducing the power losses in the power output devices.
For analog inputs, a class D amplifier operates by first converting the input into a modulated digital signal, which then is amplified, and then is filtered to recover an analog output signal. In the class D amplifier, only the digital signal is amplified by on/off digital signal processing, thus, class D amplifiers can have very high power efficiency since they provide substantially full output power, while minimizing internal power consumption.
FIG. 1 shows a prior art typical class D amplifier, comprising a comparator 13, a sawtooth waveform generator 12, a class D switching stage 15, a filter 17 and a load 19. An input Vi 10 is first compared (in the comparator 13) with a sawtooth comparator signal 11 (generated by the sawtooth generator 12) to generate a digital modulated signal 14. The modulated signal 14 is composed of a series of pulses, with the width of each pulse being proportional to the amplitude of the input voltage Vi. The modulated signal 14 is then used to drive a class D switching stage 15, generating a modulated digital signal 16. After passing through the low-pass filter 17, the digital signal 16 is converter back to an analog signal Vo 18, with amplified magnitude, to drive a load 19. A class D amplifier typically includes the class D switching stage 15 and the filter 17.
One important component of class D amplifiers is the modulation circuitry. A popular class D modulator, Pulse Width Modulation (PWM), provides a constant frequency pulse signal whose pulse width varies proportionally to the input signal. Other class D modulation techniques can also be used, such as pulse density modulation (PDM), or pulse frequency modulation (PFM). An indispensable component of class D amplifiers is the low-pass filter, typically an LC-type low-pass filter.
There are basically two class D topologies, a half-bridge with 2 output devices and a full-bridge with 4 output devices. The half-bridge design is simpler and more flexible, but can exhibit a “bus pumping” phenomenon where supply current sinks back to the power supply from the amplifier. Such changes in current direction are problematic for Low Dropout (LDO) voltage regulators, which can source current but are usually incapable of sinking current. Any standard LDO that is presented with a reverse (sink) current rather than a source current will respond by shutting off, and its output voltage will rise dramatically, usually above the LDO's input voltage. LDO “regulation” is then lost, resulting in the complete loss of the class D amplifier's output voltage regulation.
Various prior art conventional solutions exist to eliminate the problems associated with class D reverse current. However, these solutions are not universally accepted since no solution is suitable for all design limitations. An obvious prior art solution is the use of a battery to power the class D amplifier where the reverse current operates to charge the battery. The design limitation of this solution is that not all class D amplifier topologies are intended to operate from the unregulated battery voltage, relying instead on a well-controlled voltage (LDO) source. Another solution is the incorporation of a large capacitor capable of recovering the energy going back into the power supply. The major drawback of this solution is the extra space needed since a large capacitor takes up a significant real estate in a system floor plan. Another solution is to take advantage of any other power requirements in the overall system by supplying the class D amplifier power from the same power source. For example, if the rest of the system consumes 20 mA, and if the maximum reverse current is only 10 mA, then a power supply supplying the total power will always see a net drain of current, and thus will be stable. A drawback of this solution is the vagaries of system load currents. One more solution is to simply add a resistor between the output of the power supply and ground, whose value sinks a current larger than the anticipated reverse current from the class D amplifier. The main disadvantage of this approach is that the LDO must then continually source this current, wasting system power.
Still another solution is the use of differential loading. FIG. 2 shows a schematic of this solution, where one of the paths is always providing a forward current, and if the forward current always exceeds the reverse current, then the net current is always forward. The problem with this solution is that it is not an available option if the load is inherently single ended as in the case of conventional headphone. Another solution is a push-pull voltage source, shown in FIG. 3, which is a regulated voltage source (Vs) that is capable of both sourcing and sinking current. The reverse current in this voltage source is designed to be shunted to ground instead of capturing in a storage element or using for other useful purpose. The efficiency of the overall circuit is reduced somewhat due to the disposal of the reverse current. Beside the loss of efficiency, another drawback of this design is the special design of the power supply to allow the sinking of reverse current.